Pretransitional Multiscale Structure Preceding Third-Phase Formation in Solvent Extraction Systems

Third-phase formation in liquid–liquid extraction poses operational challenges by inducing organic phase separation, limiting extraction efficiency and process safety. Here, we employ combined ultrasmall-angle X-ray scattering and ultrasmall-angle neutron scattering to elucidate pretransitional structures occurring during uranium extraction by trioctylamine (TOA) in octane as the diluent. Small-angle X-ray scattering highlights uranium-filled aggregates, while SANS captures all aggregates regardless of uranium content. Extending the q -range from 0.004 to 3 Å –1 reveals two distinct populations: small, spherical, reverse micelle-like aggregates (radius ∼ 11 Å) and larger fluctuating domains exceeding 150 nm, consistent with Ornstein–Zernike critical fluctuations. Increasing the TOA concentration amplifies these nanodomains of concentrated aggregates until divergence, the formation of a third phase. This study demonstrates that phase instability originates from the hierarchical condensation of water-filled aggregates around uranium-filled aggregates rather than from simple micelle growth. Predicting such instability therefore requires accounting for the complete distribution of species, including water. Understanding such multiscale pretransitional phenomena offers new routes to predict and mitigate phase separation in solvent extraction processes critical for nuclear fuel recycling.